fix: var a{.foo.} = expr inside templates (refs #15920) (except when foo is overloaded)

[timotheecour]

fixes partially #15920

(WAS: timotheecour#89)

template foo(lhs, typ, expr) =
  let lhs = expr
proc fun1()=
  let a {.foo.} = 1
template fun2()=
  let a {.foo.} = 1
fun1() # ok
fun2() # WAS `Error: invalid pragma: foo` here

future work

• var template pragma where the template pragma is an overloaded symbol (this usually won't concern byaddr unless that symbol is overloaded)
• related to new syntax for lvalue references: var b {.byaddr.} = expr #13508 (comment)
  we should handle nkOpenSymChoice
• proc template pragma inside template still don't work

[Araq]

Yes, I think so.

[timotheecour]

left for future work, I want to get this fix in first (keeping this comment as unresolved)

[Araq]

Don't forget the [backport:1.2] tag.

[timotheecour]

@Araq please don't merge this yet, there's 1 more important thing missing that I fixed but I haven't pushed yet; if urgent, i'll do that tmrw
(related to https://github.com/nim-lang/Nim/pull/13869/files/522bbade6eee4f94647bce817edf99dc7d824590#diff-750ffc8fb636a0b3ddd159b478017510 + another unrelated thing)

[Araq]

Well it's not "urgent" but regression fixes should have some priority. :-)

[timotheecour]

@Araq PTAL, I've improved test coverage, and added disabled tests for cases that don't work yet (see "future work" in top post, plus tests in this PR)

Well it's not "urgent" but regression fixes should have some priority. :-)

but it's not a regression, this never worked before

[timotheecour]

I've reduced the single failure in important_packages (docopt) down to this:

when true:
  import macros # defines `proc genSym` symbol
  template fn() =
    var ret {.gensym.}: int
    discard ret
  fn()

=> needs fixing before this can be merged

EDIT: now fixed

[timotheecour]

@Araq PTAL, I've revisited this PR and fixed the blocker

[Clyybber]

This is a hack IMO

[timotheecour]

see test case labeled D20210801T100514